The present invention relates to active substances suitable for use as positive electrodes of nonaqueous electrolyte secondary cells, electrodes having the active substances, nonaqueous electrolyte secondary cells incorporating the electrode, and a process for fabricating the active substances.
Nonaqueous electrolyte secondary cells comprise heretofore positive and negative electrodes capable of absorbing and desorbing lithium and nonaqueous electrolytic solution. The nonaqueous electrolyte secondary cells incorporate positive electrode active substances which are fabricated from lithium-containing metal oxide which combines lithium with metals such as nickel, cobalt, and the like. Active research has been conducted on such cells since the cells can exhibit voltage output of approximately 4 V and have large cell capacity.
However, the conventional nonaqueous electrolyte secondary cells described above have the problem that when the cell is allowed to store, the positive electrode reacts with the electrolytic solution, degrading storage characteristics. To overcome this problem, JP-A No. 9-245787, for example, proposes a positive electrode active substance containing SO4 trace. Nevertheless, the nonaqueous electrolyte secondary cell incorporating the proposed positive electrode active substance still has the problem of becoming lower in discharge capacity giving the unsatisfactory storage characteristics to the cell.
An object of the present invention which has been accomplished in view of the foregoing drawbacks of the prior art is to provide active substances which, when used as the positive electrode active substances of nonaqueous electrolyte secondary cells, diminishes the reduction of its discharge capacity that would result if the cell is allowed to store, the active substances thus being suitable to give the improved storage characteristics to the cell.
Another object of the present invention is to provide an electrode which, when used as the positive electrode for a nonaqueous electrolyte secondary cell, diminishes the reduction of its discharge capacity that would result if the cell is allowed to store, the electrode thus being suitable to give the improved storage characteristics to the cell.
Still another object of the invention is to provide a nonaqueous electrolyte secondary cell which diminishes the reduction of its discharge capacity that would result when allowed to store, the nonaqueous electrolyte secondary cell thus being suitable to give the improved storage characteristics to the cell.
Further another object of the invention is to provide a process for fabricating an active substance which, when used as the positive electrode active substance of a nonaqueous electrolyte secondary cell, diminishes the reduction of its discharge capacity that would result if the cell is allowed to store, the process for fabricating an active substance thus being suitable to give the improved storage characteristics to the cell.
The present invention provides an active substance which is characterized in that the active substance contains a composition of LiNiaCobMcO2 (a+b+c=1;0xe2x89xa6cxe2x89xa60.5) wherein M is at least one metal selected from among Mn, Fe, Zn, Ti, Cr, Mg, Al, Cu, or Ga and =a composition of AlX(SO4)2 wherein X is at least one material from among Na, K, Rb, Cs, or NH4.
The active substance thus constructed, when used as the positive electrode active substance, exhibits small self-discharge rate to afford the improved storage characteristics to the cell. This is thought attributable to the fact that the active portion of LiNiaCobMcO2 is reduced suppressing the reaction between the positive electrode active substance and the electrolytic solution to restrain degrading of the positive electrode. According to an amount of M of LiNiaCObMcO2, the charge and discharge capacity may be reduced, so that the added amount of M is preferably within the range of 0xe2x89xa6cxe2x89xa60.5.
Further the active substance of the invention which contains at least 1.5 mole % to up to 20 mole % of AlX(SO4)2 based on the amount of said LiNiaCobMcO2 is used as a positive electrode active substance of nonaqueous electrolyte secondary cells giving the cell greatly reduced self-discharge rate.
Still the active substance of the invention which contains at least 3 mole % to up to 10 mole % of AlX(SO4)2 based on the amount of said LiNiaCobMcO2, is used as a positive electrode active substance of nonaqueous electrolyte secondary cells giving the cell much more greatly reduced self-discharge rate.
Further the present invention provides construction wherein surface of said LiNiaCobMcO2 is coated with said AlX(SO4)2 suppressing sufficiently the reaction between LiNiaCobMcO2 and the electrolytic solution.
The electrode of the present invention is further characterized in that the electrode is prepared using the active substance described. In the case where the electrode thus constructed is used as the positive electrode of the nonaqueous electrolyte secondary cell, the self-discharge rate of the cell becomes small to give the improved storage characteristics to the secondary cell.
The nonaqueous electrolyte secondary cell of the present invention is further characterized in that the cell is prepared using the positive electrode as described above. With the nonaqueous electrolyte secondary cell thus constructed, the self-discharge rate becomes small to give the improved storage characteristics.
The present invention provides a solute of an electrolytic solution of the nonaqueous electrolyte secondary cell such as LiPF6, LiBF4, LiSbF6, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, LiCF3(CF2)3SO3, and the like to be used. However, these examples are not limitative.
The present invention further provides a solvent of an electrolytic solution of the nonaqueous electrolyte secondary cell such as an ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, sulfolane, 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and the like to be used. However, these examples are not limitative.
A process for fabricating the active substance of the invention is characterized in that LiNiaCobMcO2 (a+b+c=1;0xe2x89xa6cxe2x89xa60.5) wherein M is at least one metal selected from among Mn, Fe, Zn, Ti, Cr, Mg, Al, Cu, or Ga and AlX(SO4)2.12H2O wherein X is at least one material from among Na, K, Rb, Cs, or NH4 are mixed together to conduct heat-treatment to the mixture obtained.
According to the above process for fabricating the active substance, the active substance described is fabricated preferably. Especially when the active substance which is produced under the temperature of at least 100xc2x0 C. to up to 300xc2x0 C. in the above heat-treatment is used as a positive electrode active substance of the nonaqueous electrolyte secondary cell, the self discharge rate of the cell becomes greatly smaller.
Embodiments of the invention will be described below in detail.
[Preparation of Positive Electrode Active Substance]
AlK(SO4)2.12H2O was mixed with LiCoO2 in the ratio of 5 mole % based on an amount of LiCoO2. Thereafter the mixture obtained was subjected to the heat-treatment at 250xc2x0 C. for 2 hours to prepare a positive electrode active substance.
[Preparation of Positive Electrode]
The above positive electrode active substance, carbon powder for use as an electrically conductive agent, and polytetrafluoroethylene serving as binder were mixed together in the ratio by weight of 80:10:10 to obtain a positive electrode composition. Subsequently, the mixture was molded under pressure and dried in a vacuum at 100xc2x0 C. for 2 hours to prepare a positive electrode.
[Preparation of Negative Electrode]
Lithium-aluminum alloy was blanked in predetermined dimension to prepare a negative electrode.
[Preparation of Electrolytic Solution]
An electrolytic solution was prepared by dissolving LiPF6 in the ratio of 1 M in a solvent mixture of ethylene carbonate and diethyl carbonate in the ratio by volume of 1:1.
[Assembly of Cell]
A positive-electrode can and a negative-electrode can each of which has a flat cylindrical shape and has an opening at one end were fabricated. While the positive electrode described above was placed on a bottom inside the positive-electrode can, the negative electrode described above was placed on a bottom inside the negative-electrode can. A separator which was impregnated with the above electrolytic solution was interposed between the negative electrode and the positive electrode, and the positive-electrode can and the negative-electrode can were interconnected by an insulator, whereby a typical structure of a coin-type cell AO was prepared. A finely porous membrane of polypropylene having ion permeability was used as the separator.